The present invention relates to a miniature solenoid valve (electrically-controlled pneumatic valve) whose design opens up the possibility of manufacturing methods that are of low cost and that guarantee high quality performance.
Progress in pneumatic automation is associated with miniaturization of the components used, and in particular of the solenoid valves that pilot directional control valves. Thus, over the last few years, manufacturers have gone from solenoid valves having a smallest dimension greater than 20 mm to valves in which said dimension is down to 10 mm. Nevertheless, manufacturing a valve of such small size requires solutions that are expensive but which enable only poor performance to be achieved because of difficulties encountered in obtaining precision dimensions for the parts making up the functional elements, and because of difficulties in the procedures for assembling them so as to obtain quality that is constant from one valve to another. Reducing dimensions has the effect of reducing fluid flow sections and thus of reducing the pneumatic power available at the outlet from the solenoid valve, and it is found in many cases that the sum of manufacturing tolerances and assembly tolerances on the functional parts of a valve of such dimensions is of the same order of magnitude as one of the dimensions of the flow sections.
Furthermore, electromagnetically, very little power can be fed to such devices because the winding is highly miniaturized. The working stroke of the plunger core of the electromagnet for driving the valve member(s) is then very short.
As a result of these constraints, it is necessary for the component parts of the valve to be manufactured with very great care and/or for the assembly procedure to be complex, including precise and individual adjustment of each valve, thereby penalizing cost price. In this respect, mention can be made of document FR 2 643 370 which illustrates one of those expensive technological solutions.
For the record, mention is also made of various other documents such as DE 2 340 304, DE 1 871 835 U, DE 4 125 816, US 3 303 854, and FR 1 198 043 all of which describe solenoid valves of structure similar to that of the invention, but not adapted to being miniaturized on an industrial scale and at acceptable cost.
The present invention seeks to remedy those drawbacks by proposing a solenoid valve of structure that is simple to manufacture and assemble, thereby giving rise to a significant reduction in cost, with manufacture being constant in quality, and with it being possible to obtain pneumatic power that is greater than required merely for piloting a directional control valve.
Another object of the invention which is achieved by the very design of the valve is to constitute a member for controlling small actuators directly. Such small actuators consume pneumatic fluid at rates which require internal flow sections in the members controlling them that correspond to a diameter of about 1 mm, which is much greater than the flow section required for controlling a directional control valve.
To this end, the invention provides a miniature solenoid valve comprising:
a valve body;
an electromagnet having a yoke secured to the body and a plunger core movable relative to the yoke along a longitudinal axis of the electromagnet;
a pneumatic chamber formed in the body adjacent to the end of the plunger core that faces away from the yoke, into which chamber there opens out freely a first duct and, each via a respective seat, second and third ducts, with the openings of the second and third ducts facing in opposite directions and being coaxial about the longitudinal axis of the electromagnet; and
first and second valve members facing the respective seats with clearance, the first valve member being carried by the plunger core and the second valve member being carried by moving equipment held in contact with the plunger core by a resilient return member so as to be moved together with the plunger core between a first position corresponding to the electromagnet being in the excited state in which the plunger core is in abutment against the yoke, and a second position corresponding to the absence of electromagnetic excitation in which the first valve member bears against its seat, the two seats being provided at respective ends of a single seat-carrier piece which includes internally the ends of the second and third ducts, said piece being engaged in the body of the valve along the common axis of the electromagnet and the two seats, and being immobilized in a precise position along said axis and specific to each valve.
This first disposition is what makes it possible industrially to ensure proper positioning of the seats relative to the stroke of the electromagnet and in particular relative to the most effective portion of said stroke, as can be seen from the description below.
In a first embodiment, the above-mentioned equipment has a set of spacers extending between the valve members, outside said seat-carrier piece, the length of the spacers being equal to the distance between the two seats plus the length of the stroke desired for each of the valve members. Insofar as the material of the second valve member is deformable, it can be preferable for the face of the second valve member facing its seat to be surmounted by a peripheral washer for bearing against the spacers, the length of the spacers being shortened to compensate by an amount equal to the distance between the bearing surface for the spacers and the surface of the active portion of the valve member.
This structure ensures that the pneumatic stroke is well controlled, with control depending on the relationship between the dimensions of the seat-carrier piece and of the spacers. By choosing to make these members out of a metal and to fabricate them by machining (with an automatic lathe), they can be obtained at relatively low cost with dimensions that are very precise. In addition, direct contact is eliminated between the metal spacers and the valve members which are necessarily made of a deformable elastomer-based material, avoiding any embedding of the spacers into the surface of the elastomer material since, in operation, that gives rise to randomness in the size of the stroke which it is desired to keep constant, and also to randomness in the relative positioning of the moving elements, particularly if the pilot forces or the resistive forces to be overcome by the valve members vary. It will be observed that this washer introduces an additional tolerance to be kept under control in order to define the pneumatic stroke, but that this is not a problem since the way the washer is fabricated (cutting) means that tight tolerances can be complied with.
In order to change the stroke, i.e. in order to change the internal flow section of the valve, only the seat-carrier piece needs to be changed, so the same control is achieved over precision, regardless of the chosen section.
In a second embodiment, the seat-carrier piece and the moving equipment are made of thermoplastic material implemented in the form of a preassembled cartridge comprising: said piece, the moving equipment with the second valve member, a stopper added to the seat-carrier piece beside the second valve member, and a resilient return member placed between the stopper and the second valve member; the moving equipment being formed by a fork of thermoplastic material comprising a cup for holding the second valve member and integrally extended by at least two longitudinal branches parallel to the above-mentioned common axis, passing through the seat-carrier piece and having free ends situated on either side of the seat situated facing the first valve member.
It will be observed that under such circumstances, the pneumatic stroke is determined during manufacture of the cartridge and in simple manner, as described below.
One of the numerous advantages of this structure lies in the fact that in a single solenoid valve body as used for implementing a standard interface with external devices, it is possible to install seat-carrier pieces of different fluid flow sections and of different lengths in order to define different strokes that match the desired flow rates, thus making it possible for the manufacture of valves of different calibers to be extremely standardized. In this context, it is recalled that in order to maintain a flow section between seat and valve member for given duct diameter, it is necessary for the stroke of the valve member to be equal to one-fourth of the diameter. It will thus be understood that if it is desired to double the flow diameter, and thus double the stroke, an increase in caliber requires only that the stroke be increased by a value equal to one-fourth of the smaller caliber diameter, and that it is easy to provide a pneumatic chamber in the valve body (e.g. made of plastic) where the chamber has the same dimensions for all calibers capable of containing the maximum stroke, with the parameters that govern how the stroke is determined being achieved by parts that are independent of the valve body and that are fitted thereto.
Among other features of the invention that are described below, mention is made of the feature whereby the seat-carrier piece is a body of revolution provided on the outside with three spike means that are axially spaced apart from one another for fixing the piece in the body of plastics material, and two outer grooves between the spike means forming sections of the second and third ducts between their terminal portions formed inside the piece and their portions formed inside the body, said sections being isolated from each other and from the pneumatic chamber. This piece is assembled to the valve body inside the pneumatic chamber merely by forcing one into the other. This, together with the fact that the portion of the body forming the side wall of the pneumatic chamber is provided with an orifice that opens out firstly into the zone of the chamber containing the nearest seat of the valve and secondly to the outside surface of the body opposite from the surface for receiving a manual control member, leads to a first method of manufacturing the valve that is particularly easy while nevertheless making it possible to obtain all of the precision that is required for a high quality solenoid valve that operates reliably.
In order to manufacture a solenoid valve having a seat-carrier piece and moving equipment that are made of metal and turned, the method consists, when assembling the valve, in inserting a piece of shim via the lateral orifice for housing the manual control member prior to the manual control member being installed, the shim being placed in front of the electromagnet and being of a thickness that is equal to the stroke desired for the valve members, in forcing the seat-carrier piece into the pneumatic chamber by means of a tool inserted via the end opening thereof prior to the stopper being put into place until the plunger core of the electromagnet is brought into abutment against its yoke, and in withdrawing the piece of shim. It is thus ensured that the stroke of the valve members corresponds with precision to the most effective stroke of the electromagnet, i.e. the stroke that comes closest to a zero airgap, thus providing better certainty that operation will take place properly at low power or low pneumatic flow rate, while also making it possible to implement high power variants (i.e. at high flow rate) when they are necessary.
In this respect, it is recalled that for given excitation power, the force developed by the electromagnet increases as the plunger core comes closer to the yoke, and thus makes it possible to overcome better the forces due to the pneumatic pressure acting on the section of the seats. Unfortunately, these forces depend on the section of the seat and thus vary as a function of the desired flow rate, such that for a given electromagnet, the opposing forces it needs to overcome differ depending on the intended application of the solenoid valve. The assembly method outlined above serves to guarantee a manufacturing procedure that puts the electromagnet in its best operating condition, whatever the purpose to which the valve would be put in the future, and thus regardless of the length of the stroke of the valve members.
To manufacture solenoid valves in which the seat-carrier piece and the moving equipment are prepared as a cartridge, the invention provides a second method which consists in assembling the cartridge, in calibrating the axial distance measured between the free ends of the branches of the fork and the seat situated facing the first valve member and in proceeding with controlled forced engagement of the cartridge into the body of the valve until accurate spacing is obtained between the first valve member and its seat when the plunger core is bearing against the yoke.
In this case, said calibration is performed by partially melting the seat and the ends of the branches of the fork by means of a shaping tool pressed hot against the corresponding end of the cartridge.
Finally, and preferably, in order to obtain the said precise spacing, a reference level of power is fed to the electromagnet while the cartridge is being forced into the body of the valve, the reference level of power tending to attract the plunger core against the yoke, and the application of force is stopped when the first valve member is observed to separate from its seat.